Legend:

I simply un-installed Hyper-V and it works again. I would like to change this to a suggestion that VirtualBox should please warn you that another virtualisation service/application is running. This will it so much clearer why x64 Guests would not start up.

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I simply un-installed Hyper-V and it works again. I would like to change this to a suggestion that VirtualBox should please warn you that another virtualisation service/application is running. This will it make it so much clearer why x64 Guests would not start up.